In the complete combustion of common gaseous fuels, the fuel combines with oxygen to produce carbon dioxide, water and heat. There can be intermediate reactions producing carbon monoxide and hydrogen. The heat, however, can also cause other chemical reactions such as causing atmospheric oxygen and nitrogen to combine to form oxides of nitrogen or NO.sub.x. While NO.sub.x may be produced in several ways, thermal NO.sub.x is associated with high temperatures, i.e. over 2000.degree. K. The flame is zoned so that different parts of the flame are at different temperatures. NO.sub.x production can be reduced with the lowering of the peak flame temperature. The reduction in NO.sub.x is required because it is a prime component in the generation of photochemical smog and reduction can be achieved through turbulence of the gases being combusted and/or by heat transfer from the high temperature portion of the flame. Inserts have been placed in the inlet portions of the heat exchanger tubes facing the inshot burners of a fuel fired heating appliance such as a furnace. Associated with the burning process in a furnace environment and with structure to reduce NO.sub.x are problems with objectionable sound generation. U.S. Pat. No. 5,146,910 discloses a NO.sub.x reducing devices which "perform their NO.sub.x reducing functions without generating an appreciable amount of noise during operation of their associated furnace".